Wireless communications devices such as the BlackBerry™ by Research in Motion Limited enable users to download map content from web-based data sources such as BlackBerry Maps™, Google Maps™ or Mapquest™. Downloaded map content is displayed on a small LCD display screen of the wireless communications device for viewing by the user. The user can pan up and down and side to side as well as zoom in or out. With the increasing availability of wireless communications devices having onboard Global Positioning System (GPS) receivers for providing location-based services (LBS), the efficient delivery and handling of map data is increasingly important.
When raw map data (i.e. “real” coordinates of latitude and longitude for all vertices of geographical features) are received at the wireless device, the map data corresponding to naturally curved geographical features has to be projected onto a display, which is a planar surface. Map projections, like Mercator, attempt to correct for the exaggerated characteristics brought about by equal degrees of latitude and longitude being rendered as though they both represent equal distances in both the x- and y-directions (a condition that is only true at the equator). In reality, the further from the equator you go, the smaller the distance between degrees of longitude become, while the distance between degrees of latitude stay the same. Geographic features thus get stretched horizontally more and more as you move away from the equator. For example, while purely east-west roads are not distorted, those with both a north-south component and an east-west component can appear distorted. For example, in the northerly (or southerly) latitudes, roads that are in fact perpendicular may appear to intersect at an angle.
Mercator projection can be used to correct for this distortion by essentially making the lines of latitude get closer together the further you get from the equator by exactly the same amount that the lines of longitude get closer together. This is known as a conformal projection, one in which the x- and y-scales are always the same. But a real projection like this involves correcting every one of its vertices individually using its “own” latitude coordinate, which is too computationally intensive to do in real-time on a small handheld device. In other words, these computationally intensive map-projection algorithms are generally unsuitable for use on small wireless handheld devices because of the limited onboard processing capacity of these devices. Accordingly, a technique for efficiently compensating for map projection distortions on wireless communications devices thus remains highly desirable.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.